feat: completed solutions

exercises/13_error_handling/errors6.rs

@@ -25,7 +25,9 @@ impl ParsePosNonzeroError {
     }
 
     // TODO: Add another error conversion function here.
-    // fn from_parse_int(???) -> Self { ??? }
+    fn from_parse_int(err: ParseIntError) -> Self {
+        Self::ParseInt(err)
+    }
 }
 
 #[derive(PartialEq, Debug)]
@@ -43,7 +45,7 @@ impl PositiveNonzeroInteger {
     fn parse(s: &str) -> Result<Self, ParsePosNonzeroError> {
         // TODO: change this to return an appropriate error instead of panicking
         // when `parse()` returns an error.
-        let x: i64 = s.parse().unwrap();
+        let x: i64 = s.parse().map_err(ParsePosNonzeroError::from_parse_int)?;
         Self::new(x).map_err(ParsePosNonzeroError::from_creation)
     }
 }